This invention relates to an optical head in which a light beam for reading, writing or erasing information is converged on an information recording medium such as an optical disk.
In the usual optical head the beam waist of a convergent light beam has to be formed on a light-reflecting surface or recording surface of the information recording medium, i.e., optical disk, when writing information on the disk, reading information therefrom or erasing information thereon. To this end, the optical head is provided with a focus servo control system for focus control such that the beam waist of a light beam is always held on the light-reflecting surface or recording surface of the optical disk. In such a focus servo optical system, particularly the optical system disclosed in the U.S. patent application Ser. No. 399,872 filed in July, 1982, now U.S. Pat. No. 4,516,331 which adopts a so-called knife edge process of detecting focus from the movement of a beam spot formed on the surface of a light detector provided in an image formation plane, on which an image concerning a light-reflecting surface or recording surface of an optical disk is formed, departure of the beam waist of a convergent beam from the light-reflecting surface or recording surface of the optical disk is liable to result from even a slight error of the length of the light path of the light beam or a slight error of the axis of the optical system that would be caused by changes in environmental conditions, particularly ambient temperature changes.